Computer disc drives are known in the art. Such systems include a number of coaxially arranged discs. Each disc is coated with a thin film of magnetic storage media. Each disc further includes a number of concentric tracks. Each track is divided into a number of sectors; data is stored in the sectors on each track.
For each disc, there is a magnetic head assembly, supported by an arm, which is selectively positioned over tracks on the disc to access the data. The width of the magnetic head is approximately equal to the width of a track.
In addition to reading and writing a user's data on the disc, the head is utilized as a position transducer component in a servo mechanism. Head position information is embedded directly on the tracks of each disc at the beginning of each sector. As the disc is rotated, the head reads the position information and transmits this information to a track position detector for processing. The actual position of the magnetic head is compared with its desired position. The difference is processed to generate a signal which is fed to a DC motor which re-positions the head at a corrected position. This servo procedure is repeated until the actual position of the head equals the desired position.
This radial positioning of the head assembly is typically undertaken in two steps. First, the head assembly seeks a particular track on the disc. This is achieved by processing position information associated with each sector of each track. That is, previously embedded sector information is read and processed as the head moves across the tracks of the disc. This step results in a rough positioning of the assembly relative to a desired track. In order to read a particular sector located on the track, in the next step, the head assembly is precisely positioned with respect to the target track. This is achieved by reading "A" and "B" servo bursts from their position centered half way between the center lines of data tracks adjacent to and on either side of the target track. The "A" and "B" bursts are processed and the head position is adjusted such that the signal from the "A" burst equals that from the "B" burst. When the signals are equivalent, the read head is positioned on the track center line. This precise alignment is maintained until it is necessary to move to another track.
A problem associated with this technology is that the servo mechanism requires precise processing of all track position data. This is particularly necessary when the data is stored on discs in a high density configuration. However, as a result of vibrations, spindle bearing run-out and other factors, when writing track position information, the servo writer is not as accurate as necessary. Whole tracks of magnetic patterns may be misplaced slightly from their proper position both radially and circumferentially.
Radial misplacements may be controlled adequately with precision servowriter mechanics and position control systems. On the other hand, for high density magnetic patterns, circumferential mispositioning may result in adjacent tracks being written entirely out of phase with one another. Thus, as the head crosses between tracks, the two tracks may destructively interfere with one another, thereby resulting in a composite signal of undetectably small amplitude. In this case, position information is not gathered and the operation of the servo mechanism is seriously disrupted.
This problem is not readily solved. As track bit densities increase, achieving track-to-track coherence requires more exacting tolerances on mechanical parts. The requisite tolerances are difficult to obtain. Similarly, the problem is difficult to solve by adjusting variables such as non-repeatable bearing run-out, head skew angle, or timing jitter during servo pattern writing.